1. Field of the Invention
This invention relates to an electronic sphygmomanometer and, more particularly, to an electronic sphygmomanometer for measuring blood pressure by electronically detecting the tapping sounds of arterial blood flow, or so-called Korotkoff sounds.
2. Description of the Prior Art
An electronic sphygmomanometer of the above type detects the Korotkoff sounds by a sensitive microphone or pressure sensor and applies the detected signal to a filter and amplifier unit of the required band width to obtain a signal of the kind shown in FIG. 1, in which A and B denote the electrical pulses representative of the systolic and diastolic Korotkoff tapping sounds, respectively. It is general practice to establish a certain threshold level with respect to the signal shown in FIG. 1 and to adopt an arrangement in which only pulses that exceed the threshold level are recognized.
In general, there is neither an abrupt increase in the magnitude of the Korotkoff pulse signal near the systolic pressure, nor is there an abrupt decrease in magnitude near the diastolic pressure. As a result, there are likely to be errors in reading the systolic and diastolic pressures, which errors are ascribable to the threshold level, as well as great differences in signal strength depending upon the individual whose blood pressure is being measured. Hence the accuracy of the sphygmomanometer is decided by the threshold level setting, or by the setting of the amplifier gain. Errors in the values measured by the prior-art electronic spyhgmomanometer occur to a marked degree in the vicinity of the diastolic pressure where the difference in signal level with respect to noise is small, and generally there is poor correlation with stethoscopy applied to detect the diastolic pressure.
Accordingly, since the magnitude of the Korotkoff sound signal differs depending upon the individual or in accordance with the conditions of measurement, a system has previously been proposed in Japanese patent application Laid-Open No. 55-122534 which takes into account the fact that errors in blood pressure measurement occur when a fixed level is employed for discrimination purposes. In the previously proposed arrangement, a circuit is provided, apart from a Korotkoff sound detecting circuit, for discriminating the Korotkoff signal level by means of a prescribed threshold level, and an AGC (automatic gain control) system is used to lower amplifier gain or to switch the threshold level when the Korotkoff signal level exceeds a predetermined value. With such an arrangement, however, amplifier gain for recognition of systolic pressure is the same as that for recognition of diastolic pressure if the input Korotkoff signal is small in magnitude or the signal level of the first detected arterial pulse is smaller than the predetermined value. Discrete levels thus cannot be set to conform to the differences which appear in the Korotkoff sound waveform at the systolic and diastolic pressures, and it is difficult to obtain a correct correlation with the measurement of diastolic pressure by means of stethoscopy.
With the above-described electronic sphygmomanometer, amplifier gain is lowered or the threshold level raised in response to a high Korotkoff signal level, but this is often a disadvantage since the gain for recognition of the diastolic pressure is not always raised to a value higher than that for recognition of the systolic pressure, thereby causing the diastolic pressure to be measured as being higher than the correct value (as measured by stethoscopy).